Antigen specific T-cell factors are most likely the solubilized T-cell antigen receptor, since it is unlikely that the T-cell has two different antigen specific molecules, the factor and a different receptor. Therefore, antigen specific T-cell factors offer a direct way to study the structure of the T-cell antigen receptor. In addition, the immunoregulatory functions of these factors provide a way of studying specific B-cell activation and T-B cooperation. Structurally, we hope to determine (a) whether the antigen specific T-cell factor bears the same idiotype as immunoglobulin (the antigen specific B-cell product), (b) if the Ia antigens constitute a portion of the factor, and (c) what molecules bind the factor to the B-cell surface and specifically whether they are related to either the Ia antigens and/or the Fc receptor. Functionally, we would like to determine (a) whether the factor, in high concentration, will lose its antigen specificity and become nonspecific, and (b) if the antigen specific T-cell enhancing and suppressing factors are identical but reflect different biologic functions of the factor under different conditions.